This class provides a skeletal implementation of the Collection
interface, to minimize the effort required to implement this interface.

To implement an unmodifiable collection, the programmer needs only to
extend this class and provide implementations for the iterator and
size methods. (The iterator returned by the iterator
method must implement hasNext and next.)

To implement a modifiable collection, the programmer must additionally
override this class's add method (which otherwise throws an
UnsupportedOperationException), and the iterator returned by the
iterator method must additionally implement its remove
method.

The programmer should generally provide a void (no argument) and
Collection constructor, as per the recommendation in the
Collection interface specification.

The documentation for each non-abstract methods in this class describes its
implementation in detail. Each of these methods may be overridden if
the collection being implemented admits a more efficient implementation.

toArray

Returns an array containing all of the elements in this collection. If
the collection makes any guarantees as to what order its elements are
returned by its iterator, this method must return the elements in the
same order. The returned array will be "safe" in that no references to
it are maintained by the collection. (In other words, this method must
allocate a new array even if the collection is backed by an Array).
The caller is thus free to modify the returned array.

This implementation allocates the array to be returned, and iterates
over the elements in the collection, storing each object reference in
the next consecutive element of the array, starting with element 0.

toArray

Returns an array containing all of the elements in this collection;
the runtime type of the returned array is that of the specified array.
If the collection fits in the specified array, it is returned therein.
Otherwise, a new array is allocated with the runtime type of the
specified array and the size of this collection.

If the collection fits in the specified array with room to spare (i.e.,
the array has more elements than the collection), the element in the
array immediately following the end of the collection is set to
null. This is useful in determining the length of the
collection only if the caller knows that the collection does
not contain any null elements.)

If this collection makes any guarantees as to what order its elements
are returned by its iterator, this method must return the elements in
the same order.

This implementation checks if the array is large enough to contain the
collection; if not, it allocates a new array of the correct size and
type (using reflection). Then, it iterates over the collection,
storing each object reference in the next consecutive element of the
array, starting with element 0. If the array is larger than the
collection, a null is stored in the first location after the
end of the collection.

add

Ensures that this collection contains the specified element (optional
operation). Returns true if the collection changed as a
result of the call. (Returns false if this collection does
not permit duplicates and already contains the specified element.)
Collections that support this operation may place limitations on what
elements may be added to the collection. In particular, some
collections will refuse to add null elements, and others will
impose restrictions on the type of elements that may be added.
Collection classes should clearly specify in their documentation any
restrictions on what elements may be added.

remove

Removes a single instance of the specified element from this
collection, if it is present (optional operation). More formally,
removes an element e such that (o==null ? e==null :
o.equals(e)), if the collection contains one or more such
elements. Returns true if the collection contained the
specified element (or equivalently, if the collection changed as a
result of the call).

This implementation iterates over the collection looking for the
specified element. If it finds the element, it removes the element
from the collection using the iterator's remove method.

Note that this implementation throws an
UnsupportedOperationException if the iterator returned by this
collection's iterator method does not implement the remove
method and this collection contains the specified object.

containsAll

Returns true if this collection contains all of the elements
in the specified collection.

This implementation iterates over the specified collection, checking
each element returned by the iterator in turn to see if it's
contained in this collection. If all elements are so contained
true is returned, otherwise false.

addAll

Adds all of the elements in the specified collection to this collection
(optional operation). The behavior of this operation is undefined if
the specified collection is modified while the operation is in
progress. (This implies that the behavior of this call is undefined if
the specified collection is this collection, and this collection is
nonempty.)

This implementation iterates over the specified collection, and adds
each object returned by the iterator to this collection, in turn.

Note that this implementation will throw an
UnsupportedOperationException unless add is
overridden (assuming the specified collection is non-empty).

removeAll

Removes from this collection all of its elements that are contained in
the specified collection (optional operation).

This implementation iterates over this collection, checking each
element returned by the iterator in turn to see if it's contained
in the specified collection. If it's so contained, it's removed from
this collection with the iterator's remove method.

Note that this implementation will throw an
UnsupportedOperationException if the iterator returned by the
iterator method does not implement the remove method
and this collection contains one or more elements in common with the
specified collection.

retainAll

Retains only the elements in this collection that are contained in the
specified collection (optional operation). In other words, removes
from this collection all of its elements that are not contained in the
specified collection.

This implementation iterates over this collection, checking each
element returned by the iterator in turn to see if it's contained
in the specified collection. If it's not so contained, it's removed
from this collection with the iterator's remove method.

Note that this implementation will throw an
UnsupportedOperationException if the iterator returned by the
iterator method does not implement the remove method
and this collection contains one or more elements not present in the
specified collection.

clear

Removes all of the elements from this collection (optional operation).
The collection will be empty after this call returns (unless it throws
an exception).

This implementation iterates over this collection, removing each
element using the Iterator.remove operation. Most
implementations will probably choose to override this method for
efficiency.

Note that this implementation will throw an
UnsupportedOperationException if the iterator returned by this
collection's iterator method does not implement the
remove method and this collection is non-empty.

toString

Returns a string representation of this collection. The string
representation consists of a list of the collection's elements in the
order they are returned by its iterator, enclosed in square brackets
("[]"). Adjacent elements are separated by the characters
", " (comma and space). Elements are converted to strings as
by String.valueOf(Object).

This implementation creates an empty string buffer, appends a left
square bracket, and iterates over the collection appending the string
representation of each element in turn. After appending each element
except the last, the string ", " is appended. Finally a right
bracket is appended. A string is obtained from the string buffer, and
returned.

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